1. Field of the Invention
The present invention relates to a copper alloy composed of fine grains whose form and orientation are controlled, and to a method of manufacturing the same.
This application claims priority from Japanese Patent Application No. 2004-118968 filed on Apr. 14, 2004, the disclosure of which is incorporated by reference herein.
2. Background Art
As described in Japanese Patent Application, First Publication No. 2002-356728, there has hitherto been known a technique of refining grains, which includes subjecting a base metal including a copper alloy to a rolling treatment and an aging treatment thereby to disperse fine precipitates, using a rolling method after subjecting to a solution treatment, and subjecting to intensive working thereby to accumulate high-density strain in the base metal and to cause low temperature dynamic recrystallization (also referred to as dynamic continuous recrystallization).
When pure copper and a copper alloy are subjected to the above intensive working using such a technique, heat is generated during working to cause recovery or recrystallization, and thus it is difficult to accumulate desired strain in the base metal. Because the resulting work is thermally unstable after working, elongation of the copper alloy is improved by subjecting to an aging treatment or a strain relief annealing, while the strength tends to decrease.
In contrast, the copper alloy containing Zr changes the entire situation when subjected to the above-mentioned intensive working. When a base metal comprising a copper alloy containing Zr is subjected to intensive working, heat generated during working is less likely to cause recovery or recrystallization, thus making it possible to accumulate desired strain in the base metal. However, when the base metal comprising a copper alloy containing Zr is subjected to intensive working after it was once precipitated, the copper alloy exhibited less improvement in elongation.
In the case of comparing with the copper alloy obtained by forming precipitates after intensive working, it is inferior in stress relaxation resistance, and in spring properties. FIG. 8 is a schematic view showing an example of the precipitation state of a Cu—Zr based compound. As is apparent from FIG. 8, Cu—Zr based precipitates 83 are commonly formed at grain boundaries. Therefore, it is considered to be more effective for the Cu—Zr based precipitates 83 to be formed after increasing the surface area of grain boundaries 82 by refining grains 81 as compared with the case wherein grains 81 are refined after forming Cu—Zr based precipitates 83. In FIG. 8, the symbol 80 denotes a visual field of a microscope.
In addition, a copper alloy containing a high concentration of Ti, Ni, or Sn is used as a base metal having high work hardenability. However, such a copper alloy had a problem that intensive working is hardly conducted and productivity is low. It is known that, in a copper alloy containing a high concentration of Zr, excess Zr segregates at grain boundaries, thereby deteriorating plating properties.
It is known that, when the above-mentioned rolling method is applied to a copper alloy and the copper alloy is rolled at a rolling reduction of not greater than 90%, grains have a large grain size and the copper alloy exhibit small elongation even in the case of a copper alloy containing Zr which heat generated during working is less likely to cause recovery or recrystallization, let alone in the case of a copper alloy free from Zr. Not only in the case of a copper alloy free from Zr also in the case of a copper alloy containing Zr, an intensity ratio of crystal orientation {110}<112> to random orientation was less than 10, and an intensity ratio of crystal orientation {112}<111> to random orientation was greater than 20, as shown in FIG. 6.
Examples of the method for working treatment of a copper alloy include ECAP (Equal Channel Angular Pressing) method described in FURUKAWA, HORITA, NEMOTO, TG. Landon: Metal, 70, 11 (2000), pp. 971; ARB (Accumulative Roll Bonding) method described in NISHIYAMA, SAKAI, SAITO: Journal of the JRICu, 41, 1 (2002), pp. 246; Mechanical Milling method described in TAKAGI, KIMURA: Material, 34, 8 (1995), pp. 959; and multiaxis/multistage working method described in Preliminary Manuscript of 42nd Lecture of Japan Research Institute for Advanced Copper-Base Materials and Technologies, pp. 55; in addition to the above-mentioned rolling method.
Using the methods disclosed in the above documents, the copper alloy is subjected to a working treatment, thus making it possible to refine grains. However, since fine grains having a grain size of not greater than 1 μm are uniformly formed by these methods, a surface area of the grains drastically increases as compared with a conventional crystal structure, which leads to large stress relaxation due to grain boundary diffusion under the environment at high temperature higher than room temperature, thus resulting in poor stress relaxation resistance. When employing these methods, it was very difficult to reconcile an improvement in strength due to grain refinement, and stress relaxation resistance.
As described above, when the strength of the copper alloy is increased by the rolling method, a technique of increasing the rolling reduction has conventionally been employed. When the rolling reduction is set to a high value, the strength of the copper alloy increases, while the elongation decreases and bendability tends to deteriorate. Therefore, it has been desired to develop a copper alloy which is excellent in three respects, for example, strength, elongation, and bendability, and a method of controlling a crystal structure with excellent stress relaxation resistance.